Investigators are searching for Heda Umarova, pictured here with Boston Marathon bombing suspect Dzhokhar Tsarnaev in an image on a Russian social networking site, after she failed to return from a family trip to Chechnya last year. The others pictured have been blurred out to protect their identity.

U.S. counter-terrorism officials are attempting to track down a female friend of the accused Boston Marathon bomber after she traveled to Chechnya last year and is believed to have since posted "alarming" jihadi imagery online, officials told ABC News.

Officials are concerned that Heda Umarova, 23, may have been radicalized to Islamist violence -- allegedly just like her friend Dzhokhar Tsarnaev, charged in the April 15 bombings, and his brother Tamerlan, who was killed in a police shootout -- and could pose a threat to Americans overseas because her U.S. passport allows easy foreign travel.

Umarova left Boston with her family in July with a round-trip ticket to visit relatives in their native Chechnya, but she failed to return to Massachusetts with her parents at the end of August. Her family told federal authorities and ABC News she stayed behind to get married after she met a man there during Ramadan.

Her younger brothers, Adam, 20, and Junes, 18, were already under suspicion by some investigators because of their online support for the Tsarnaev brothers and social media postings that included tweets with Dzhokhar a day before the blasts. A photo of Junes Umarov and Dzhokhar Tsarnaev lighting fireworks appeared on one social networking site in January 2013, an ABC News investigation found.

"No one is calling Heda a terrorist but her travel has certainly garnered some attention. People are concerned that a 23-year-old is in Chechnya, a country that she fled from... and now she is deciding to stay on her own," a ranking law enforcement source involved in the Boston Marathon investigation told ABC News. The source said that her decision to stay in Chechnya also raised eyebrows because, at least until she is married, she didn't seem to have any "obvious means" of support.

"We were already concerned about the social media exchanged with her brothers, who remain on the radar, especially the younger brother Junes, who was Dzhokhar's best friend," the source added.

And now, sources said, the FBI has been monitoring Heda Umarova's apparent Internet posts recently, which include depictions of Chechen jihadis brandishing weapons and a photo-shopped image of a U.S. passport in a carry-on bag bearing the black flag of jihad.

Heda and her brothers did not respond to multiple attempts by ABC News to reach them through their parents and social media to comment on the sources' allegations.

In a brief interview at his family's home last month, Heda's father Hamzat Umarov told ABC News that he spoke to the "FBI and CIA" about the photographs but refused to talk about whether he believes his daughter is becoming radicalized.

"What does it mean, pictures? Pictures can be anywhere. It doesn't mean anything, the pictures. We don't want to talk about it. We talk with the FBI, everybody. The FBI came, CIA, everybody's come,'' he said.

Umarova's trip back to the same country from which her parents fled as refugees a decade ago to seek political asylum in the U.S. brought federal agents back to her family's Chelsea home -- the same second-floor apartment that armed agents searched last April during the manhunt for the accused marathon bomber, several law enforcement officials told ABC News.

Heda Umarova has not been indicted for any offenses, sources said.

A spokesman for the Boston FBI field office declined to comment on Heda Umarova or any possible threat she may pose. "It is the FBI's policy not to confirm or deny whether or not an investigation is being conducted,'' said Special Agent Gregory Comcowich.

Heda Umarova's Sister: We Were Treated Like 'Terrorists'

The Umarovs were questioned by immigration officials extensively about the reasons for the return trip to Chechnya when they left Boston last summer, scrutiny that only intensified when they returned without Heda, law enforcement sources said.

"They had a hard time getting out of the country and an even harder time coming back when she [Heda] was not with them,'' said a law enforcement official familiar with the Umarov family.

The questioning led Heda's sister, Hawa Umarova, 26, to complain to U.S. Customs and Border Protection and Boston Joint Terrorism Task Force officials that her family was treated like "terrorists," the sources said, despite their constant cooperation with various local and federal law enforcement agencies.

Chelsea Police Chief Brian Keyes confirmed that Hawa Umarova was cooperative with his officers last April after the identities of the suspected marathon bombers became known and that her family submitted to a voluntary search of their home during the desperate search for Dhzokhar Tsarnaev.

Federal investigators were led to Chelsea initially by Twitter messages exchanged between Dzhokhar Tsarnaev and Junes Umarov a day before the twin blasts exploded along the finish line of the Boston Marathon, killing three and wounding another 260 others last April 15.

In addition to the social media exchanges between Junes and Dzhokhar, federal authorities are now also scrutinizing a Russian social networking page using the name Heda Umarova that was linked to a fan page for Dzhokhar on the same social networking site.

The page is sympathetic to Tsarnaev and his supporters, who insist on his innocence. It features several pictures of the Umarov siblings with Dzhokhar Tsarnaev, including one with Heda, her brother Adam, and "Johar" as a boy, which is the Russian spelling of the accused terrorist's nickname.

But the postings on the Heda Umarova VK page that have concerned federal counter-terrorism officials include photographs of several women dressed as jihadi fighters in Chechnya, taken probably about a decade ago, an expert said.

In one photo, a woman in a black headscarf is toting an AK-47 rifle. Other extremist postings support martyrdom and violence for Islam.

Days after ABC News spoke with Heda’s parents, the images in question and the link to the fan page for Dzhokhar Tsarnaev disappeared from the Heda Umarova VK page.

The social networking page also included links to Kavkaz Center, which is a jihadist media portal linked to Doku Umarov, the jihadi leader of the Caucusus Emirate who threatened to strike the Sochi Olympics last July. There is no familial relation to the terror leader and Umarov is a common name, family members in Chelsea, Mass. told ABC News.

Investigators found a YouTube page created by Tamerlan Tsarnaev that included a lecture given by a leader connected to Doku Umarov, law enforcement officials said, but there is no evidence the North Caucasus militant leader influenced the marathon attacks.

Heda's Mother: I 'Love' the US

During another interview last week, Heda's mother, Raisa Umarova, became emotional when asked why her daughter stayed behind in Chechnya, a country that she and her husband desperately wanted to leave 10 years ago when their five children were "babies."

"She is getting married. She doesn't speak good Russian. They [the Umarov children] come to this country as babies. This is my home now. I love this country. I love my children being here," the mother said through tears.

Raisa Umarova said her family came to Massachusetts as political refugees but became proud U.S. citizens after seven years here.

The Umarovs as new immigrants became friendly with the Tsarnaevs in 2004, three years after the Tsarnaevs arrived, Hamzat Umarov told federal officials, and acknowledged to ABC News that the families "knew each other."

Both families were natives of Chechnya -- though the elder Tsarnaevs now live in neighboring Dagestan -- who successfully sought political asylum in the United States by citing the staggering violence in that region and "the Tsarnaevs took them under their wing," an investigator said.

But despite her younger children's public support for Dzhokhar, Raisa Umarova insisted that she and her husband have no contact with the Tsarnaev family.

"I don't like them. I like my country,'' she said, referring to America.

No one in the Umarov family has been charged with a crime or named as a suspect complicit in the April 15 attack on the Boston Marathon.

Chelsea High School officials said federal investigators interviewed Junes' teachers and classmates after the marathon bombings last year.

Chelsea High Principal Joseph Mullaney told ABC News all five of the Umarov children attended the public school and called them "bright students." Junes and Adam were in Advanced Placement classes, like their older sister Hawa, who speaks several languages, Mullaney added.

Hawa went on to graduate from Mass College of Liberal Arts in North Adams, Mass. Junes and Adam are roommates and students at MCLA now. Adam Umarov was questioned on campus last year, officials said.

Chelsea High School librarian Thelma Dakubu said the entire Umarov family strongly identified with Chechnya. Adam even posted the Chechen flag of Ichkeria – which symbolizes the ongoing conflict between the Russian federation and nationalists there – on his Chelsea High School Yearbook page in 2012.

Dakubu was also quick to add that the family appeared tight-knit and hardworking. The family patriarch, Hamzat Umarov, even performed a custom Chechen dance at a high school talent show with his daughters, she said.

"They seem to be a close family,'' Dakubu said.

In his 2013 high school yearbook, Junes Umarov wrote, "I'm a stress free kind of guy" -- which is the identical quote Dzhokhar Tsarnaev tweeted two days after the bombings, one of several he sent during the chaotic search for the attackers.

Both Hamzat Umarov and his wife Raisa Umarova insisted in separate interviews that Heda stayed in Chechnya to get married. Raisa Umarova said that the future groom is "lovely" and that Heda met him during Ramadan services during the family trip.

The family, however, would not provide details on the wedding and declined to identify the groom to ABC News. Sources said law enforcement officials were similarly stonewalled.

Tsarnaev has been held in a Massachusetts federal prison at Fort Devens since his arrest and the Department of Justice has announced prosecutors will seek the death penalty. Tsarnaev is also charged with the assassination of MIT Police Officer Sean Collier, who prosecutors said was gunned down by Tamerlan and Dzhokhar Tsarnaev on April 18.

U.S. District Court Judge George A. O'Toole ruled last month that the trial is slated to begin in November and allowed the defense team to add another death penalty attorney to work on the case. Last week defense attorneys complained in a court filing that the FBI is monitoring every file that they review with Tsarnaev, endangering his right to a fair trial. Much of the court filings on the case are under seal.

Michele McPhee is a freelance reporter and frequent ABC News contributor based in Boston.